The increasing demand for ever smaller and more complex microstructured devices and the continuing demand for an increase of a throughput in the manufacturing and inspection processes thereof have been an incentive for the development of particle-optical systems that use multiple charged particle beamlets in place of a single charged particle beam, thus significantly improving the throughput of such systems. The multiple charged particle beamlets may be provided by a single column using a multi-aperture array, for instance, or by multiple individual columns, or a combination of both, as will be described in more detail below. The use of multiple beamlets is associated with a whole range of new challenges to the design of particle-optical components, arrangements and systems, such as microscopes, lithography systems, and mask repair systems.
A conventional multiple charged-particle multi-beamlet system is known from U.S. Pat. No. 6,252,412 BI. The electron microscopy apparatus disclosed therein is used for inspecting an object, such as a semiconductor wafer. A plurality of primary electron beams is focused in parallel to each other on the object to form a plurality of primary electron spots thereon. Secondary electrons generated by the primary electrons and emanating from respective primary electron spots are detected. For each primary electron beam a separate electron beam column is provided. The plurality of separate electron beam columns is closely packed. A density of the primary electron beam spots formed on the object is limited by a remaining footprint of the electron beam columns forming the electron microscopy apparatus. Thus, the number of primary electron beam spots, which may be formed simultaneously on the object, is also limited in practice, resulting in a limited throughput of the apparatus when inspecting semiconductor wafers at a high resolution.
From U.S. Pat. No. 5,892,224, US 2002/0148961 A1, US 2002/0142496 A1, US 2002/0130262 A1, US 2002/0109090 A1, US 2002/0033449 A1, US 2002/0028399 A1, electron microscopy apparatuses are known which use a plurality of primary electron beamlets focused onto the surface of the object to be inspected.
From WO 2005/024881, a particle-optical system is known, wherein a plurality of beamlets are generated by illuminating a multi-aperture plate having a plurality of apertures formed therein with a single electron beam generated by an electron source provided upstream of the multi-aperture plate. The electron beamlets are formed downstream of the multi-aperture plate by those electrons of the electron beam that traverse the apertures. The plurality of primary electron beamlets is focused on the object by an objective lens having a bore, which is traversed by all primary electron beamlets. An array of primary electron spots is then formed on the object. Secondary electrons emanating from each primary electron spot form a respective secondary electron beamlet, such that a plurality of secondary electron beamlets corresponding to the plurality of primary electron beam spots is generated. The plurality of secondary electron beamlets also traverse the objective lens, and the apparatus provides a secondary electron beam path such that each of the secondary electron beamlets is supplied to a respective one of a plurality of detectors. A Wien-filter is used for separating the secondary electron beam path from a beam path of the primary electron beamlets. Since one common primary electron beam path comprising the plurality of primary electron beamlets and one common secondary electron beam path comprising the plurality of secondary electron beamlets is used, irradiation and imaging of an object can be performed simultaneously in a plurality of sub-regions within the field of view thus enhancing the throughput of the system by reducing the inspection or processing time.
Increasing the number of primary beamlets used can thus increase the throughput of such a system. The maximum possible number of beamlets within a utilizable field of view is defined by the minimum beamlet pitch achievable, which in turn is directly related to the resolution of the secondary electron optical system. The latter is primarily a function of the electric field strength present within the space between the sample surface and the objective lens of the particle-optical system, which functions as an extraction field for the secondary electrons. An increase in the strength of the extraction field generally improves the lateral (transverse to the system's optical axis) resolution of the secondary electron-optical system.
The electrical field strength above the sample surface can however not be adjusted at will, since many samples tolerate only limited electrical field strengths at their surface. Electrical fields of higher strength can result in an unwanted modification of the sample's surface configuration to the point of damaging the as sample. Since the extraction field strength at the sample surface is to a certain amount determined by the field penetration from components of the system's charged-particle optical column, a changing of the extraction field over a wide range would require an adaptation of the column's mechanical setup and of the potentials applied to its electro-optically operative components. Since the potentials within the particle-optical column are usually predetermined, respective changes of the charged-particle optical column are not possible. Accordingly, the part of the extraction field strength defined by the penetration from components of the system's charged-particle optical column forms a restricted parameter, so that the extraction field strength can generally be changed only within a limited range.
The maximum possible number of primary beamlets and thus the throughput of the system can therefore not be optimized to applications of the charged-particle optical system, where samples are used, which allow higher extraction field strengths.
It is therefore desirable to have features available in charged-particle optical systems enabling an effective and easy to manage adaptation of the system to a particular application.